What it takes from the circuit
The block puts 37.5 kΩ across whatever it measures, so it always draws a little current: 0.24 mA from a 9 V battery. A battery or a supply does not notice. A source with resistance of its own does, and reads low.
Measuring costs a little current
A multimeter set to volts draws almost nothing from what it measures. This block is not a multimeter. It is two resistors across the terminal, 37.5 kΩ in all, and current flows through them for as long as anything is connected:
| On the terminal | Current it draws |
|---|---|
| 5 V | 0.13 mA |
| 9 V | 0.24 mA |
| 12 V | 0.32 mA |
| 25 V | 0.67 mA |
That current comes from the thing you are measuring, not from your board. From a battery it is a small, steady drain. It continues when your board is off, because the block needs no power of its own to draw it.
A source that has resistance
Every source has some resistance inside it. A battery's is a fraction of an ohm, and 37.5 kΩ against a fraction of an ohm changes nothing you can measure.
Some sources have much more. A sensor's output, a divider somebody else built, a thermistor circuit: their own resistance sits in series with the block's 37.5 kΩ and forms a second divider. The terminal sees less than the source is putting out, and the sketch reports that lower number faithfully.
At 1 kΩ the reading is about 2.6 % low, more than the resistors' own tolerance. At 10 kΩ it is a fifth low. No calibration factor fixes it properly, because the error changes with the source. The fix is to not put a divider there: a voltage already under your board's supply goes straight to the analog pin.
When to disconnect it
A battery left screwed to the terminal is a battery being drained at a quarter of a milliamp, all the time. A 9 V alkaline cell holds a few hundred milliamp-hours, so the block alone flattens it in months. For a project that watches a battery for weeks, that is part of the budget; for one in a drawer, unscrew it.
When it does not work
Slowly, if you leave it connected. It draws 0.24 mA from 9 V, day and night, whether your board is running or not. A 9 V alkaline battery holds a few hundred milliamp-hours, so the block alone empties it in a matter of months. Unscrew the battery when the project is put away.
The sensor's output probably has resistance of its own, and the block's 37.5 kΩ divides it again before the terminal sees it. If the output is already under your board's supply voltage, skip this block and wire it to the analog pin directly. If not, it needs a buffer, an op-amp follower, in front.
It depends on the source. Against a battery or a bench supply, which have a fraction of an ohm inside them, it is enormous and changes nothing. Against a sensor with a 10 kΩ output it is barely four times larger, and the reading is a fifth low.
A little. Seen from the analog pin, SIG has the two resistors in parallel behind it, 6 kΩ. The Uno's datasheet asks for 10 kΩ or less for accurate readings, so it is inside that.
The rule that decides whether you can measure something at all.
It measures from GND →Edit this page — content/books/voltmeter/what-it-takes-from-the-circuit.mdx
Questions about this product
See what other owners have asked, and read their solutions.
Voltmeter
Loading discussions…
Discuss this article
Ask about this page. The answer stays here, on the page it belongs to, for whoever hits the same wall next.